A synchronous motor is constituted by a rotor provided with permanent magnets and a wound stator supplied with A.C. power so as to generate a rotating field that drives the rotor. The speed of the rotor is equal to that of the rotating field. In a synchronous generator, the rotation of the rotor provided with permanent magnets induces the alternating currents within the windings of the stator.
The stator windings of an autosynchronous motor are supplied by a D.C. power source chopped by controlled switches, such as transistors or thyristors, the control being such that the rotating field has an optimal position with respect to that of the rotor in order to obtain a maximum-value motor torque. In order to render the phase of the stator currents dependent upon the position of the rotor, use is made of a device for determining the position of the rotor relative to that of the stator. This position determining device generates one pulse per rotation of the rotor and the phase of the stator current is rendered dependent upon the instant that this pulse appears.
The position determining device comprises, for example, a code wheel driven by the rotor and a steady or fixed detector, for example a luminous source-light detector assembly, the whole being disposed in such a way that the pulse is generated once a pole of the magnet with which the rotor is provided has reached a determined position (with respect to the position of the detector) relative to the stator. This device comprises, in the present example, two other code wheels that both transmit 2.sup.m pulses per rotation, m being the number of poles of the rotor. These two other code wheels detect the direction of rotation and the pulses produced are used in order to generate the order signals for the controlled switches, i.e. for determining the phase of the currents supplying the stator.
Each position determining device has a fixed or first part integral with the stator and a moveable or second part integral with the rotor. The first part must have a position that is determined with respect to the windings of the stator and the second part must have a position that is determined with respect to the poles of the rotor. If the position of the first part of the position determining device is modified with respect to the stator, the proper operation of the motor can be impaired as the rotating field may not have the optimal direction relative to the poles of the rotor.
At starting, since the position of the rotor is unknown, the stator is supplied by an independent generator in order to work step by step. Only after the pulse issued from the first code wheel has been transmitted, can the current phase in the stator windings can be rendered dependent upon the position of the rotor in order to produce the maximum torque.
It will be observed that for certain applications it is objectionable that upon starting, the motor rotates step by step, i.e with jerky movements. Furthermore, for reasons of space, it is often inconvenient that the part of the position determining device secured to the stator has a fixed position with respect to said stator.
The present invention overcomes these drawbacks.